Assuming the anti-vax parents' children socialize with other anti-vax parents' children the parents are playing Russian roulette pointing the gun at their children's head.
If an anti-vax kid is isolated from other anti-vax kids that kid is protected by the 'halo' effect. None of the kids around that kid have it to give.
People tend to associate with others who share their interests and beliefs.
Hence the recent spate of serious and repeated outbreaks of diseases that were until recently almost completely eradicated in the developed world.
When I was a kid, the adults all remembered how awful it was before vaccines. Most knew someone who had died. There was no question as to the benefits of being vaccinated.
Nowadays many parents have never seen a child with measles. So they are able to believe the morons who claim that it's a harmless and natural disease, and is far preferable to an unnatural and apparently needless medical intervention.
That it appears needless because it's widespread and effective never occurs to these idiots.
It's like saying 'why do cars need all these safety features, hardly anyone dies in a car crash these days'. Well, duh. Check out the casualty rates from the 1950s.